The present invention relates to a laser interferometric measuring machine, and particularly to one that is less affected by disturbance, reasonable in price and highly accurate.
In order to achieve the resolving power of 0.1 .mu.m or the higher resolving power than that for measurement in a conventional interferometric measuring machine, there is employed a method wherein a gas laser or a semiconductor laser is used as a light source, and its beam is split into a reference beam and a measuring beam and thus measurement (displacement) information is obtained by observing interference fringes and beats after aligning the beam that returns from an object to be measured with the reference beam to be coaxial again.
All interferometers are based on the type of a Michelson interferometer shown in FIG. 17 in terms of constitutional arrangement. Namely, a beam from light source 20 is split into a reference beam and a measuring beam by beam splitter 21, and the measuring beam that reflects on movable mirror 22 attached on an object to be measured is aligned by beam splitter 21 to be coaxial with the beam reflected on fixed mirror 23, thus, interference fringes are generated. Depending on a method for detecting measurement information, there are two different systems; one is an interference fringe counting system wherein the number of density changes in an interference fringe generated by a returned beam is counted and the other is a heterodyne system wherein a plurality of coherent light having different wavelengths are used to cause a beat and a frequency fluctuation of the beat is detected utilizing a Doppler shift of frequency of the measuring beam caused by the moving speed of the object to be measured.
Optical arrangement of the former system mentioned above is shown in FIG. 27, and it is described in detail in the known documents written by M. J. Dowhs and K. W. Raihel including detected signals processing. Namely, in the laser-based measuring machine mentioned above, a linearly polarized beam emitted from stabilized laser 101 is split by beam splitter 102 into a reference beam path including .lambda./8 plate 103 as well as fixed corner-cube 104 and a measuring beam path including movable corner-cube 105. The reference beam split to follow the reference beam path, after passing through .lambda./8 plate 103 twice, is converted to a circularly polarized beam which is aligned again by beam splitter 102 to be coaxial with a linearly polarized beam of a measuring beam that is split to follow the measuring beam path, and then is separated into two. One of the two beams is further split by polarized beam splitter 106 into two so that the measuring beam becomes separated beams each being in the directions of .+-.45.degree. against the polarization surface of the measuring beam. Three kinds of separated beams obtained in the aforesaid manner, after they pass through filter 107 and polarization plate 108, are converted to three kinds of interference fringe intensities whose phases, which interfere with the movement of movable corner-cube 105 in the arrowed direction, are deviated by 90.degree. in succession, and these three kinds of interference fringe intensities enter detector 109 respectively to be converted to three kinds of electric signals each having a phase difference of 90.degree.. After these three kinds of electric signals are amplified by amplifier 110 respectively, they are grouped into two groups each having two adjacent electric signals differing in terms of phase by 90.degree.. A change in the length of the measuring beam path is obtained from at least one of the electric signals changing with Sin .theta. and Cos .theta. (wherein, .theta.=2.pi.(Lm-Lr)/.lambda., Lm . . . length of measuring beam path, Lr . . . length of reference beam path, .lambda.. . . wavelength) deviated by 90.degree. in terms of phase and obtained through the input of electric signals of two groups into subtracter 111, and a direction of the change is obtained from both of the electric signals.
Namely, in this laser-based measuring machine, three kinds of interference fringe signals differing in terms of phase by 90.degree. in succession are obtained through utilization of a principle of polarization of a laser beam, and from the difference of signals between two adjacent and successive interference fringes, the signals which are used for measurement of a change and a direction of the change in the length of a measuring beam path and change with aforesaid Sin .theta. and Cos .theta.. Therefore, influence of disturbance such as a fluctuation in intensity of a laser beam is offset and thereby the center of a signal level is kept constant continually, resulting in less erroneous counting of interference fringes and higher accuracy of measurement. Owing to this, accurate counting of interference fringes is realized even when the disturbance affects the intensity of a counting beam to be changed by about 90%. Furthermore, the interference fringe counting system may use the optical arrangement as shown in FIG. 18 in which a circularly polarized light is used as a light source. In this case, the same interferometric measuring device as that of the latter one of the heterodyne system may be used. Namely, linearly polarized light emitted from stabilized laser 20A passes through .lambda./4 phase shifter 24 where it is converted to a circularly polarized beam, and then is split by polarized beam splitter 21 into a reference beam and a measuring beam. The measuring beam reflects off of movable mirror 22 and returns to polarized beam splitter 21, during the course of which the measuring beam, when it passes through .lambda./4 phase shifter 25 twice, is converted to the beam whose polarization directions cross at right angles. This is reflected off of polarized beam splitter 21. Then, it is reflected off of fixed mirror 23 and is aligned to be coaxial with the reference beam which has passed polarized beam splitter 21 after passing through .lambda./4 phase shifter 25 twice and being converted in terms of polarization direction. Here, no interference is caused because the polarization direction of the reference beam and that of the measuring beam cross at right angles. However, when common polarization component is extracted, through beam splitter 26, by polarizers 27 and 29, interference fringes are generated. The number of density changes in an interference fringe thus generated is detected by detectors 28 and 30, and in this case, polarizers 27 and 29 are arranged so that polarized beams crossing at right angles can be extracted for the purpose of discriminating the moving direction of movable mirror 22.
Optical system of the latter system mentioned above is shown in FIG. 19, and it is described in detail in the known documents PRECISION ENGINEERING Vol. 5. No. 3 (1983) 111 written by L. J. Wuerz and R. C. Quenelle. Namely, a beam emitted from 2-frequency Zeeman laser 20B generating two circularly polarized beams having respectively frequency f1 and frequency f2 and rotating reversely each other is changed by the .lambda./4 phase shifter 24 to a linearly polarized beam having planes of polarization crossing at right angles to each other. After passing through the beam splitter 34 and the polarizer 31, a beat (f1-f2) caused by frequencies f1 and f2 is detected by detector 32. A beam reflected on movable mirror 22 is subjected, due to a Doppler effect, to a frequency change .delta.f corresponding to a moving speed, and thereby the beat detected by detector 33 is based on (f1-f2.+-..delta.f). Thus, displacement information of an object can be obtained through the comparison subtraction between beat frequencies detected by both detectors 32 and 33. Incidentally, due to polarized beam splitter 35, beam f1 and beam f2 are detected respectively by detector 36 and detector 37 and the detected results are inputted in laser synchronization circuit 38.
In both examples, the resolving power for measurement covering even electrical processing is as extremely high as 0.01 .mu.m, and for the purpose of maintaining this resolving power at a highly accurate level, the frequency of laser light source is stabilized within an accuracy of 10.sup.-6 -10.sup.-8.
Further, as an example of how to stabilize detected signals against disturbance in a heterodyne system, there is known a method having an interferometric measuring machine provided with an optical path of a differential type shown in FIG. 20.
In FIG. 20, a measuring beam takes the path shown with a solid line of polarized beam splitter 21.fwdarw..lambda./4 phase shifter 25.movable mirror (prism) 22.fwdarw.polarized beam splitter 21.fwdarw.prism 40.fwdarw.polarized beam splitter 21.fwdarw..lambda./4 phase shifter 25.movable mirror 22.fwdarw.polarized beam splitter 21.fwdarw.mirror 43.fwdarw..lambda./4 phase shifter 25.prism 41.fwdarw.mirror 43.fwdarw.polarized beam splitter 21.fwdarw.prism 39.fwdarw.polarized beam splitter 21.fwdarw.exiting.
A reference beam takes the path shown with a dotted line of polarized beam splitter 21.fwdarw.prism 39.fwdarw.polarized beam splitter 21.fwdarw.mirror 41.fwdarw..lambda./4 phase shifter 25.prism 41.fwdarw.mirror 43.fwdarw.polarized beam splitter 21.fwdarw..lambda./4 phase shifter 25.fixed mirror (reflecting mirror) 42.fwdarw.polarized beam splitter 21.fwdarw.prism 40.fwdarw.polarized beam splitter 21.fwdarw..lambda./4 phase shifter 25.fixed mirror 42.fwdarw.polarized beam splitter 21.fwdarw.exiting.
In this way, it is possible to stabilize detected signals remarkably by making the length of an optical path of a reference beam and that of a measuring beam to be the same in an interferometric measuring machine wherein a beam of frequency-stabilized laser is split into a reference beam and a measuring beam, because the disturbance such as a temperature change or the like affects both beams equally.
As is apparent from FIG. 20, however, a conventional interferometric measuring machine of a differential type is extremely complicated in structure and large in size, and the length of an optical path in measuring prism is markedly long, which acts as a negative factor for enhancing the stability against disturbance.
Actually, the resolving power for measurement is 5 nm in the example shown in FIG. 20.
Further, as shown in FIG. 21, there is a method wherein a high stability peculiar to an interferometer of a differential type is realized by decreasing the number of optical elements and shortening the length of an optical path in the interferometer as shown in FIG. 21.
Similarly to the one shown in FIG. 20, the method in FIG. 21 is of a heterodyne system wherein two laser beams having different frequencies (or wavelengths) and linear polarization crossing at right angles are caused to enter polarization shearing plate 50. Then, the beam that is a linearly polarized beam of S-component selectively reflects repeatedly and thus takes a separate optical path shifted from the optical path of a P-component beam. In the known example, an amount of the shift is about 12 mm.
Further, polarization of the S-component beam is converted to a P-component type after it passes through .lambda./2 phase shifter (.lambda./2 wavelength plate, .lambda./2 phase plate) 51, thus, both beams of different wavelengths are caused to enter polarized beam splitter 52 as a P-component. Then, both beams of different wavelengths pass through polarized beam splitter 52 and polarized circularly by .lambda./4 phase shifter (.lambda./4 phase plate, .lambda./4 wavelength plate) 53, and the reference beam and the measuring beam are caused respectively by fixed mirror (reference mirror) 54 and movable mirror (plane mirror) 55 to return taking their same optical paths. Since both returning beams pass through .lambda./4 phase shifter 53 again, they are converted to S-component polarization and reflect off of the surface 52A of the polarized beam splitter. Further, they are caused to return by corner-cube prism (or mirror) 56 to surface 52A of the polarized beam splitter and reflect from that surface to advance respectively to fixed mirror 54 and movable mirror 55 where they reflect, and then are subjected to conversion by means of .lambda./4 phase shifter 53 and exit polarized beam splitter 52 as P-component. A beam of other wavelength is converted by .lambda./2 phase shifter 51' and is shifted by polarization shearing plate 50 to the same optical path and is led to a detection optical system.
The example shown in FIG. 21, compared with the one shown in FIG. 20, has less optical elements and has an optical path that is relatively short. Therefore, its stability against disturbance is easily improved and its resolving power for measurement is 1.25 nm.
In both conventional examples in FIGS. 20 and 21 mentioned above, the stability against disturbance and measuring accuracy are enhanced in an interferometer of a differential type by making both measurement direction and reference direction the same.
Strictly speaking, however, both of them can not be regarded as a method of a perfectly differential type. The reason is that the reference beam and the measuring beam are not coaxial, being shifted by about several millimeters though they are in the same direction in both conventional examples. Even the portion from the incident point to the fixed mirror, which is generally called a common path, is not a perfect common optical path. Namely, even in such a common path, the reference beam and the measuring beam are not equally affected by disturbances such as a temperature change and air flicker, though the difference between them is small.
Therefore, even in an interferometer of a differential type similar to the conventional examples, it is an important point to shorten the optical path in the interferometer to stabilize and improve the measurement accuracy.
In the past, however, a differential type has required an optical path with a complicated deflection path, resulting in the inconsistency of a long optical path.
Even the length of the common optical path in FIG. 21 exceeds 200 mm, and it is nearly twice the length of that in an interferometer of a non-differential type. This is a problem.
In view of the situation mentioned above, the first object of the invention is to realize an inexpensive and accurate laser interferometric measuring machine wherein the length of an optical path is minimized and the number of optical elements is decreased. Thus, the accuracy for measurement against disturbance is stably maintained.
The present invention further relates to a laser-based measuring machine wherein a beam emitted from a laser is split by a splitting means into a reference beam and a measuring beam. After both beams are aligned to be coaxial, they are optically split into 2-phase or 3-phase interference beams whose phases deviate by 90 degrees. Thereby, the phase change and the direction of the change in the length of measuring optical path are measured based upon electrical signals obtained by detecting aforesaid interference beams.
As an example of the laser-based measuring machine mentioned above, there is known a laser-based measuring machine of an interference fringe-counting type shown in FIG. 27.
A conventional laser-based measuring machine of aforesaid type, however, has the disadvantage that not only does polarized beam splitter 106 need to be positioned accurately against beam splitter 102 but also each one of three kinds of separated beam paths need to be provided accurately with a filter 107 and a polarization plate 108.
The second object of the invention is to provide a laser-based measuring machine of an interference fringe-counting type wherein a plurality of optical elements, which split a beam optically into 2-phase or 3-phase interference beams, can be positioned easily and accurately. The third object of the invention is to provide a laser-based measuring machine wherein three kinds of interference fringe signals, each differing in terms of phase by 90.degree. which are for obtaining signals which are not affected by disturbance such as a fluctuation in intensity of a laser beam and change with aforesaid Sin .theta. and Cos .theta., are obtained from the signals detected from 2-phase interference beams. The fourth object of the invention is to offer a laser-based measuring machine in which it is easy to accurately adjust the phase difference to be 90.degree. between 2-phase interference beams obtained through optical splitting.